The present invention relates to thermite coatings applied to molten metal filters. The coatings involve an exothermic reaction, an oxidation-reduction reaction, or a combination thereof which occurs as the metal is filtered. The advantages provided by the use of the thermite coating on the filter are: the reduction of thermal shock to the filter, the avoidance of metal freezing during filtering, improved filtration efficiency, and the protection of the filter against chemical attack. All of these advantages may provide improved filter performance.
In the processing of molten metals, it has been found advantageous to filter the metal in the liquid state. To filter metal as a liquid requires a filter with extraordinary properties. The filter must be able to withstand extreme thermal shock, chemical corrosion, and mechanical stresses. The present molten metal filter art employs ceramic monoliths whose main components are sintered silicon carbide, zirconia, alumina and/or silica with modifiers added as required.
Generally in the working of molten metals, reduced metals are heated to above their melting points and poured into castings either for purposes of storage or for molding into a product. During the pouring operation, prior to the casting, a ceramic filter has been introduced to entrap inclusions out of the molten metal. It has been discovered by those knowledgeable in the molten metal casting art, that excluding certain contaminants from the cast provides solid metals with superior properties at reduced costs.
Certain molten metals, for example super alloys, stainless steels, steel alloys, cast irons, and nonferrous metals, are heated to temperatures which test the very limits of the physical and chemical properties of the ceramic material. That these limits are exceeded is evidenced by catastrophic failure of the filter during the pour. During a catastrophic failure the filter breaks into many pieces. The filter may experience less than catastrophic failure and still be inoperable due to some other failure mechanism. For example, if the mechanical strength of the ceramic material is exceeded, the filter may deform in the direction of flow.
The ceramic material is also subject to chemical corrosion. The molten metal slag may, by way of illustration, attack the silicon oxygen bonds in silica and thereby weaken the structural integrity of the filter. This slag attack or dissolution is a cause of significant failures in filters.
It has been found that many of the inclusions in the molten metals are oxide based impurities which are better filtered by an oxide based ceramic. Filterability can be enhanced by matching the surface energies of the oxide filter to the inclusion to be filtered. Therefore, surface effects are important to consider when choosing the most efficient filter for a particular molten metal.
Finally, some problems in filtering molten metals can be directly associated with the freezing of the molten metal as it contacts the filter. Since the filter is at temperatures significantly less than the molten metal pour temperature, the initial molten metal which contacts the filter must impart heat to the filter. Since the filter draws heat from the metal, that part of the molten metal affected decreases in temperature which may cause the metal to freeze. As the metal solidifies in the filter, the solid metal may block entirely or at least partially the filterability of the ceramic, or it will slow the rate of filtering in the initial stages of the pour, thus decreasing filter performance. This same phenomenon is realized in the several gating or entry means that are positionally placed before the molten metal filter. It has been found advantageous to coat the entry means, such as nozzles, sheathes, pouring cups or basins, sprues, sprue wells, runners, or reservoirs, with thermite coatings to avoid metal freezing prior to contact with the filter. Additionally, it may be found to be advantageous to add the thermite coating to the downstream side or exit means of the filter. A problem in the art is found to exist in molds and runner systems after the filter, since metal freezing may commonly decrease the efficiency of casting.
The invention herein disclosed solves many of the above cited problems associated with the molten metal filter art. It has been found that a thermite coating involving an exothermic reaction, an oxidation-reduction reaction, or a combination thereof on the surface of the filter, whose reaction product remains on the surface of the filter during filtering, provides operable filters which heretofore would fail in the same environment.